1. Field of the Invention
This invention relates generally to telecommunications, and, more particularly, to a method and apparatus for detecting a threshold in a line card, such as ring-trip and fault detection thresholds.
2. Description of the Related Art
In communications systems, particularly telephony, it is a common practice to transmit signals between a subscriber station and a central switching office via a two-wire bidirectional communication channel. A line card generally connects the subscriber station to the central switching office through a subscriber line. At the subscriber end, a telephonic device may be employed to establish communication with a remote user using the subscriber line. The combination of the telephonic device and the subscriber line is commonly referred to as a subscriber loop.
A line card generally includes at least one subscriber line interface circuit (SLIC) as well as a subscriber line audio-processing circuit (SLAC). The SLIC interfaces with the subscriber loop, and the SLAC interfaces with the SLIC. The SLIC and the SLAC carry out the well-known BORSCHT (Battery feed, Overvoltage protection, Ringing, Supervision, Coding, Hybrid, and Test) functions.
Typically, when an end user initiates a call, the line card provides a ringing AC ringing signal and, often, a DC bias signal, to the subscriber loop to ring the telephonic device. In the United States, the AC ringing signal generally varies from a 16 Hz to 66-⅔ Hz, although a 20 Hz signal is commonly used. Other countries may employ a ringing signal of a different frequency than that of the ringing signal employed in the United States. For example, in European countries, the ringing signal is 25 Hz. The ringing signal can either be internally or externally generated.
While applying the ringing signal to the subscriber loop, the line card also detects an off-hook condition of the telephonic device. Upon detection of an off-hook event, the line card terminates the transmission of the ringing signals within a predetermined amount of time, which is generally within 200 ms of detecting the off-hook condition. The process of transmitting a ringing signal and then detecting the switch-hook condition of the telephonic device is referred to as ring-trip detection.
Aside from ring-trip detection, line cards perform a variety of other key functions using signals of varying frequency. One such function is AC-fault detection. The purpose of AC-fault detection is to ensure that there are no undesirable interrupts caused by an AC disturbance signal, such as a power line signal or a rail system signal. For AC-fault detection, line cards employ a signal having a frequency of 16.67 Hz, 50 Hz, or 60 Hz.
To perform ring-trip detection, AC-fault detection, or other key functions, line cards generally transmit a signal to the subscriber loop and then calculate a power of a received signal. For example, ring-trip detection is determined by comparing the calculated power of the received ringing signal to a threshold value. Similarly, AC-fault detection and other line functions may also require calculation of the power of the received signal. The power calculation is usually based on a computation of one complete cycle of the received signal, which means that it is desirable to know a period (or frequency) of the received signal.
Line cards are generally designed to operate in multiple countries, and should therefore be robust enough to conform to the requirements of individual countries. Line cards should be flexible enough to comply with the different frequency requirements in different countries for functions such as ring-trip detection, AC-fault detection, and the like. And, since the power calculation is generally calculated for one period of the received signal, it becomes increasingly difficult for designers to calculate power for signals of varying frequencies.
To account for the different frequency requirements, one method employed by designers is to calculate power based on a compromise between the various frequencies. For example, for ring-trip detection, line cards utilize an integration time of 44 ms, which correlates to a signal having a frequency of 22.5 Hz, an average of a 20 Hz signal (i.e., frequency commonly employed in U.S.) and 25 Hz signal (i.e., frequency utilized in European countries). Thus, the ring-trip detection under this method is based on a period of 44 milliseconds. Likewise, for AC-fault detection, the line cards may employ an integration time of 100 ms, which is a compromise between the 50 Hz and 60 Hz signals. In some instances an integration time of 60 ms may be employed, which is a compromise between the 16.67 Hz and 50 Hz signal. This method of utilizing compromising integration times for ring-trip detection and AC-fault detection, for example, may result in at least 10% false detections.
Since signals of varying frequencies are employed in a line card for a variety of functions, the power calculation may not always be accurate. One way of improving power calculation in a line card is to allow an end user to input the exact period of the signal employed. This, however, requires not only additional hardware interface support, but also places an added burden on the end user. For instance, the end user will have to know the exact frequency, and hence, the exact period, of the signal being employed.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.